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New information on the dynamics of relativistic nucleus-nucleus collisions

Al Jipa C Besliu O Ristea C Ristea M Calin D Argintaru T Esanu I Lazanu V Covlea C Bordeianu C Iosif I Grossu A Scurtu S Velica V Baban A Birzu S Cioranu A Paduraru S Paragina F Paragina D Stoica for the BRAHMS Collaboration

University of Bucharest Faculty of Physics Bucharest Romania

E-mail oanabrahmsfizicaunibucro

Abstract Relativistic heavy-ion collisions offer a unique opportunity to study highly excited dense nuclear matter in the laboratory We present measurements of identified charged hadron production at different rapidities from Au+Au and p+p collisions at 200 GeV Coulomb effects on pion spectra in relativistic nuclear collisions at RHIC energies will be investigated The nuclear modification factors for identified particles show distinct mesonbaryon dependence At high pT the charged pion yields are suppressed by a factor of ~5 while the baryon production is enhanced in Au+Au collisions when compared to the binary scaled p+p data from the same energy

1 Introduction Heavy ion collisions at relativistic energies are used to study the properties of nuclear matter in extreme conditions of temperature and density and to analyze the possible phase transition from hadronic matter to a new state of matter called the quark and gluon plasma QGP [1-4] Since hadrons contain basic information about collision dynamics the production of hadrons is one of the important probes of QGP

Transverse momentum spectra of hadrons produced in relativistic nuclear collisions provide valuable information on particle production mechanisms as well as dynamics and properties of the matter produced The intermediate pT region is considered to have both soft and hard hadron production mechanisms The soft part includes hydrodynamic collective flow parton recombination and the hard part includes jet fragmentation and its quenching

2 BRAHMS Experiment The data presented here were collected with BRAHMS detector system [5] from RHIC (Relativistic Heavy Ion Collider) [6] BRAHMS (Broad RAnge Hadron Magnetic Spectrometers) consists of a set of global detectors for event characterization and two magnetic spectrometers the mid-rapidity spectrometer (MRS) and the forward spectrometer (FS) which identify charged hadrons over a broad range of rapidity and transverse momentum Collision centrality is determined from the charged particle multiplicity measured by a set of global detectors

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

Published under licence by IOP Publishing Ltd 1

The Mid-Rapidity Spectrometer (MRS) which operates in the polar angle interval from 90 to

30 is composed of a single dipole magnet placed between two Time Projection Chambers (TPC) and a Time-of-Flight (TOF) detector for particle identification The Forward Spectrometer (FS) which operates in the polar angle range of 15 ltθ lt 23 has two TPCs three Drift Chambers and four dipole magnets Particle identification in the FS is provided by TOF measurements in two separate hodoscopes (H1 and H2) andor by using a Ring Imaging Cherenkov detector (RICH) located at the end of the spectrometer The mid-rapidity arm is capable of separating pions from Kaons up to 2 GeVc and charged kaons from protons or antiprotons up to 3 GeVc while the forward arm can identify particles up to 35-40 GeVc by using the Cherenkov ring detector (RICH)

3 Experimental results

31 Enhancement factors One of the proposed quark-gluon plasma (QGP) signals is the increased strangeness production compared to that of a hadron gas [7] Strange particles are of particular interest since all strange hadrons must be formed in the matter produced Other processes can also enhance strangeness production [8] and therefore elementary p-p collisions where QGP formation is unlikely are important as a reference We investigate the kaon production in Au+Au collisions to gain better insight into strange quarks production The pion and kaon enhancement factors are presented in Fig 1 The enhancement factor is defined as the yield per mean number of participating nucleons Npart in Au+Au collisions divided by the respective value in p+p collisions at the same energy

⎟⎟⎠

⎞⎜⎜⎝

⎛=

++

dydN

dydN

NE

ppAA

part

2 (1)

Figure 1 Enhancement factors for negatively charged pions and kaons as a function of Npart in 200 GeV Au+Au and p+p collisions STAR data are from ref [9] Error-bars represent statistical and systematic uncertainties on the A+A measurements added in quadrature The shaded bands depict model uncertainties on number of participants calculation The bands on the left show uncertainties from the pp measurements that are correlated for all data points

An enhancement of kaon production with respect to pions as a function of collision centrality is

observed at midrapidity as well as at forward rapidity At rapidity y~3 the enhancement factors for negative charged kaons are about 2 with respect to elementary interactions The different behavior for pion and kaon enhancement factors at forward rapidity may suggest that there are different particle

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

2

production mechanisms at work as a function of rapidity Such a difference would still have to be checked in terms of modifications from pp collisions as manifestation of the isospin effect

Van Hove in ref [10] discusses the possibility that the mean-transverse momentum vs rapidity density correlation could provide a signal for QGP formation considering that the dNdy reflects the entropy and the transverse momentum spectrum is related to the temperature of the system and the transverse expansion of the hadronic matter

From the pT spectra obtained in most central 0-10 Au+Au collisions at NNs =200 GeV the particle yields are evaluated for each rapidity interval by integrating the covered pT range and extrapolating to regions outside the experiment acceptance The chosen rapidity intervals for this analysis were (-01 01) (08 09) (27 28) (28 29) (29 30) (30 31) (31 32) (32 33) (33 34) (34 35) and (35 36) The kaon spectra were fitted with an exponential formula and the resulting kaon yields were calculated by extrapolating the fit function to the full pT range The mean transverse momentum of charged kaons is calculated using intint= TTTTTT dppfdppfpp )()(

where f(pT) is the exponential function The ltpTgt - dNdy results are shown in the Fig 2

Figure 2 The mean transverse momentum as a function of dNdy for positive (left) and negative (right) kaons produced in central 0-10 Au-Au collisions at 200 GeV

The two points from y~0 and 085 may correspond to the QGP phase at midrapidity in the central

region of the collision As the rapidity increases ltpTgt and dNdy values decrease and may be interpreted in terms of formation of a mixed phase of QGP and hadrons during the evolution of the heavy-ion system The points from y~3 reflect the hadronic phase because at the projectile and target rapidities the system temperature and the particle density are both lower and we donrsquot expect QGP formation in these regions However the pT distributions of the hadrons produced do not reflect the conditions from the early stages of the collision and are influenced by collective flow

32 Coulomb interaction at relativistic energies The asymmetry in the number of charged pions produced in heavy ion collisions at AGS and SPS energies was interpreted as an effect of Coulomb interaction between the pions produced and the positive charge from reaction partners At lower energies the colliding nuclei are fully stopped and expand relatively slowly in all directions therefore the total charge stays together for sufficient time to significantly accelerate or decelerated the charged pions produced The interaction between charged pions and net charge of protons changes the transverse momentum of pions with the Coulomb ldquokickrdquo pc [11]

cT

cT

pppp

Tmm

minus+

⎟⎟⎠

⎞⎜⎜⎝

⎛ minus=

+perp

minusperp

+

minus

+

minus

expππ

ππ

(2)

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

3

where ( )22cT ppmm plusmn+=plusmn

perp At higher energies the colliding nuclei are no longer stopped the system expands faster in longitudinal direction (a higher degree of transparency) resulting in a smaller Coulomb effect

The ratio of negative to positive pions produced in Au+Au collisions at two different energies

NNs = 5 GeV (at E866-AGS) and NNs =200 GeV (at BRAHMS-RHIC) as a function of transverse mass is shown in the left panel of the Fig 3 At low mT ndash mass the pion ratios behave differently while at high mT ndash mass both ratios approach one A pronounced enhancement in the +minus ππ ratio at low transverse kinetic energy is evident for the AGS data while the BRAHMS data are found to be almost flat with respect to mT -mass If stopping is significant a large amount of positive charge from the beam and target nuclei will concentrate around the mid-rapidity of the colliding system Therefore the AGS enhancement could be explained if low-pT positive pions are pushed towards higher pT by a large net positive charge at mid-rapidity conversely negative pions would be attracted At RHIC energy a low net-baryon density is observed at midrapidity [14] and consequently there are negligible Coulomb effects

Figure 3 Left AGS Au+Au at 116 AGeVc (full symbols) [12] and BRAHMS Au+Au at 200 GeV [13] (red full symbols are for 0-10 centrality blue open symbols for 40-60 centrality) Right Charged pion ratio produced in 0-10 Au+Au collisions at 200 GeV [13] as a function of transverse momentum The lines are the calculations using the relation (2) for pc=2 MeVc (blue) pc=5 MeVc (red) and pc=8 MeVc (green)

In the right panel of Fig 3 the BRAHMS negative to positive charged pions ratio for the most central 0-10 Au+Au collisions at NNs =200 GeV is plotted as a function of transverse momentum is compared to the ratio predicted by Eq (2) using three constant values of Coulomb kick 2 MeVc 5 MeVc and 8 MeVc

33 High pT suppression The nuclear modification factor RAA for unidentified charged hadrons as function of pT for the most central Au+Au collisions at pseudorapidities 0 08 26 30 and 36 is shown in Figure 4 The error bars are statistical the shaded bands are the systematic errors The shaded band around unity shows the systematic uncertainty in the number of binary collisions The RAA values decrease for pTgt2 GeVc showing the suppression of the charged hadron yields relative to the p+p reference At high pT (pT gt4 GeVc) the charged hadron yields are suppressed by a factor of ~ 3 as compared with binary scaled p+p yields For all the studied pseudorapidities the RAA distribution remains systematically lower than unity for central collisions and shows a slight decrease of the high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This suppression has been interpreted as due to

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

4

the energy loss of the energetic partons traversing the hot and dense medium produced The high pT suppression observed in the most central 200 GeV Au+Au collisions persists over a wide range in pseudorapidity and it is almost constant for all the angles studied

The nuclear modification factors for particles identified show distinct mesonbaryon dependence Nuclear modification factors for charged pions protons and antiprotons produced in Au+Au collisions at NNs =200 GeV for five rapidities y = 00 08 26 31 34 are presented in the Figure 5 Error bars represent statistical errors The shaded boxes around points show systematic errors The dotted lines indicate the expectation of binary scaling The shaded band around unity indicates the systematic error associated with the uncertainty in the number of binary collisions

Figure 4 Nuclear modification factor RAA for unidentified charged hadrons at different pseudorapidities for 200 GeV Au+Au collisions (0-10 centrality) Figure taken from ref [16]

For all the studied rapidities the pion RAA distribution remains systematically lower than unity for

most central Au+Au collisions at NNs =200 GeV At high pT the charged pion yields are suppressed by a factor of ~ 5 as compared with binary scaled p+p pion yields The suppression of pions at high pT compared with p+p collisions indicates that the partons undergo a large energy loss due to a hot dense medium created during the collisions The RAA shows constant high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This result could be an indication that other nuclear effects than parton energy loss might contribute to the rapidity constant suppression [15]

Figure 5 Nuclear modification factor RAA for pions protons and antiprotons at different rapidity for the most central collisions (0-10 centrality) Figure taken from ref [16]

In contrast the proton and antiproton yields do not show suppression with respect to binary scaling in the intermediate pT range for all the rapidities Due to the poor statistics for rapidity y=26 which corresponds to the FS positioned at 80 relative to the beam line we present only the RAA for protons

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

5

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6

The Mid-Rapidity Spectrometer (MRS) which operates in the polar angle interval from 90 to

30 is composed of a single dipole magnet placed between two Time Projection Chambers (TPC) and a Time-of-Flight (TOF) detector for particle identification The Forward Spectrometer (FS) which operates in the polar angle range of 15 ltθ lt 23 has two TPCs three Drift Chambers and four dipole magnets Particle identification in the FS is provided by TOF measurements in two separate hodoscopes (H1 and H2) andor by using a Ring Imaging Cherenkov detector (RICH) located at the end of the spectrometer The mid-rapidity arm is capable of separating pions from Kaons up to 2 GeVc and charged kaons from protons or antiprotons up to 3 GeVc while the forward arm can identify particles up to 35-40 GeVc by using the Cherenkov ring detector (RICH)

3 Experimental results

31 Enhancement factors One of the proposed quark-gluon plasma (QGP) signals is the increased strangeness production compared to that of a hadron gas [7] Strange particles are of particular interest since all strange hadrons must be formed in the matter produced Other processes can also enhance strangeness production [8] and therefore elementary p-p collisions where QGP formation is unlikely are important as a reference We investigate the kaon production in Au+Au collisions to gain better insight into strange quarks production The pion and kaon enhancement factors are presented in Fig 1 The enhancement factor is defined as the yield per mean number of participating nucleons Npart in Au+Au collisions divided by the respective value in p+p collisions at the same energy

⎟⎟⎠

⎞⎜⎜⎝

⎛=

++

dydN

dydN

NE

ppAA

part

2 (1)

Figure 1 Enhancement factors for negatively charged pions and kaons as a function of Npart in 200 GeV Au+Au and p+p collisions STAR data are from ref [9] Error-bars represent statistical and systematic uncertainties on the A+A measurements added in quadrature The shaded bands depict model uncertainties on number of participants calculation The bands on the left show uncertainties from the pp measurements that are correlated for all data points

An enhancement of kaon production with respect to pions as a function of collision centrality is

observed at midrapidity as well as at forward rapidity At rapidity y~3 the enhancement factors for negative charged kaons are about 2 with respect to elementary interactions The different behavior for pion and kaon enhancement factors at forward rapidity may suggest that there are different particle

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

2

production mechanisms at work as a function of rapidity Such a difference would still have to be checked in terms of modifications from pp collisions as manifestation of the isospin effect

Van Hove in ref [10] discusses the possibility that the mean-transverse momentum vs rapidity density correlation could provide a signal for QGP formation considering that the dNdy reflects the entropy and the transverse momentum spectrum is related to the temperature of the system and the transverse expansion of the hadronic matter

From the pT spectra obtained in most central 0-10 Au+Au collisions at NNs =200 GeV the particle yields are evaluated for each rapidity interval by integrating the covered pT range and extrapolating to regions outside the experiment acceptance The chosen rapidity intervals for this analysis were (-01 01) (08 09) (27 28) (28 29) (29 30) (30 31) (31 32) (32 33) (33 34) (34 35) and (35 36) The kaon spectra were fitted with an exponential formula and the resulting kaon yields were calculated by extrapolating the fit function to the full pT range The mean transverse momentum of charged kaons is calculated using intint= TTTTTT dppfdppfpp )()(

where f(pT) is the exponential function The ltpTgt - dNdy results are shown in the Fig 2

Figure 2 The mean transverse momentum as a function of dNdy for positive (left) and negative (right) kaons produced in central 0-10 Au-Au collisions at 200 GeV

The two points from y~0 and 085 may correspond to the QGP phase at midrapidity in the central

region of the collision As the rapidity increases ltpTgt and dNdy values decrease and may be interpreted in terms of formation of a mixed phase of QGP and hadrons during the evolution of the heavy-ion system The points from y~3 reflect the hadronic phase because at the projectile and target rapidities the system temperature and the particle density are both lower and we donrsquot expect QGP formation in these regions However the pT distributions of the hadrons produced do not reflect the conditions from the early stages of the collision and are influenced by collective flow

32 Coulomb interaction at relativistic energies The asymmetry in the number of charged pions produced in heavy ion collisions at AGS and SPS energies was interpreted as an effect of Coulomb interaction between the pions produced and the positive charge from reaction partners At lower energies the colliding nuclei are fully stopped and expand relatively slowly in all directions therefore the total charge stays together for sufficient time to significantly accelerate or decelerated the charged pions produced The interaction between charged pions and net charge of protons changes the transverse momentum of pions with the Coulomb ldquokickrdquo pc [11]

cT

cT

pppp

Tmm

minus+

⎟⎟⎠

⎞⎜⎜⎝

⎛ minus=

+perp

minusperp

+

minus

+

minus

expππ

ππ

(2)

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

3

where ( )22cT ppmm plusmn+=plusmn

perp At higher energies the colliding nuclei are no longer stopped the system expands faster in longitudinal direction (a higher degree of transparency) resulting in a smaller Coulomb effect

The ratio of negative to positive pions produced in Au+Au collisions at two different energies

NNs = 5 GeV (at E866-AGS) and NNs =200 GeV (at BRAHMS-RHIC) as a function of transverse mass is shown in the left panel of the Fig 3 At low mT ndash mass the pion ratios behave differently while at high mT ndash mass both ratios approach one A pronounced enhancement in the +minus ππ ratio at low transverse kinetic energy is evident for the AGS data while the BRAHMS data are found to be almost flat with respect to mT -mass If stopping is significant a large amount of positive charge from the beam and target nuclei will concentrate around the mid-rapidity of the colliding system Therefore the AGS enhancement could be explained if low-pT positive pions are pushed towards higher pT by a large net positive charge at mid-rapidity conversely negative pions would be attracted At RHIC energy a low net-baryon density is observed at midrapidity [14] and consequently there are negligible Coulomb effects

Figure 3 Left AGS Au+Au at 116 AGeVc (full symbols) [12] and BRAHMS Au+Au at 200 GeV [13] (red full symbols are for 0-10 centrality blue open symbols for 40-60 centrality) Right Charged pion ratio produced in 0-10 Au+Au collisions at 200 GeV [13] as a function of transverse momentum The lines are the calculations using the relation (2) for pc=2 MeVc (blue) pc=5 MeVc (red) and pc=8 MeVc (green)

In the right panel of Fig 3 the BRAHMS negative to positive charged pions ratio for the most central 0-10 Au+Au collisions at NNs =200 GeV is plotted as a function of transverse momentum is compared to the ratio predicted by Eq (2) using three constant values of Coulomb kick 2 MeVc 5 MeVc and 8 MeVc

33 High pT suppression The nuclear modification factor RAA for unidentified charged hadrons as function of pT for the most central Au+Au collisions at pseudorapidities 0 08 26 30 and 36 is shown in Figure 4 The error bars are statistical the shaded bands are the systematic errors The shaded band around unity shows the systematic uncertainty in the number of binary collisions The RAA values decrease for pTgt2 GeVc showing the suppression of the charged hadron yields relative to the p+p reference At high pT (pT gt4 GeVc) the charged hadron yields are suppressed by a factor of ~ 3 as compared with binary scaled p+p yields For all the studied pseudorapidities the RAA distribution remains systematically lower than unity for central collisions and shows a slight decrease of the high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This suppression has been interpreted as due to

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

4

the energy loss of the energetic partons traversing the hot and dense medium produced The high pT suppression observed in the most central 200 GeV Au+Au collisions persists over a wide range in pseudorapidity and it is almost constant for all the angles studied

The nuclear modification factors for particles identified show distinct mesonbaryon dependence Nuclear modification factors for charged pions protons and antiprotons produced in Au+Au collisions at NNs =200 GeV for five rapidities y = 00 08 26 31 34 are presented in the Figure 5 Error bars represent statistical errors The shaded boxes around points show systematic errors The dotted lines indicate the expectation of binary scaling The shaded band around unity indicates the systematic error associated with the uncertainty in the number of binary collisions

Figure 4 Nuclear modification factor RAA for unidentified charged hadrons at different pseudorapidities for 200 GeV Au+Au collisions (0-10 centrality) Figure taken from ref [16]

For all the studied rapidities the pion RAA distribution remains systematically lower than unity for

most central Au+Au collisions at NNs =200 GeV At high pT the charged pion yields are suppressed by a factor of ~ 5 as compared with binary scaled p+p pion yields The suppression of pions at high pT compared with p+p collisions indicates that the partons undergo a large energy loss due to a hot dense medium created during the collisions The RAA shows constant high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This result could be an indication that other nuclear effects than parton energy loss might contribute to the rapidity constant suppression [15]

Figure 5 Nuclear modification factor RAA for pions protons and antiprotons at different rapidity for the most central collisions (0-10 centrality) Figure taken from ref [16]

In contrast the proton and antiproton yields do not show suppression with respect to binary scaling in the intermediate pT range for all the rapidities Due to the poor statistics for rapidity y=26 which corresponds to the FS positioned at 80 relative to the beam line we present only the RAA for protons

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

5

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6

production mechanisms at work as a function of rapidity Such a difference would still have to be checked in terms of modifications from pp collisions as manifestation of the isospin effect

Van Hove in ref [10] discusses the possibility that the mean-transverse momentum vs rapidity density correlation could provide a signal for QGP formation considering that the dNdy reflects the entropy and the transverse momentum spectrum is related to the temperature of the system and the transverse expansion of the hadronic matter

From the pT spectra obtained in most central 0-10 Au+Au collisions at NNs =200 GeV the particle yields are evaluated for each rapidity interval by integrating the covered pT range and extrapolating to regions outside the experiment acceptance The chosen rapidity intervals for this analysis were (-01 01) (08 09) (27 28) (28 29) (29 30) (30 31) (31 32) (32 33) (33 34) (34 35) and (35 36) The kaon spectra were fitted with an exponential formula and the resulting kaon yields were calculated by extrapolating the fit function to the full pT range The mean transverse momentum of charged kaons is calculated using intint= TTTTTT dppfdppfpp )()(

where f(pT) is the exponential function The ltpTgt - dNdy results are shown in the Fig 2

Figure 2 The mean transverse momentum as a function of dNdy for positive (left) and negative (right) kaons produced in central 0-10 Au-Au collisions at 200 GeV

The two points from y~0 and 085 may correspond to the QGP phase at midrapidity in the central

region of the collision As the rapidity increases ltpTgt and dNdy values decrease and may be interpreted in terms of formation of a mixed phase of QGP and hadrons during the evolution of the heavy-ion system The points from y~3 reflect the hadronic phase because at the projectile and target rapidities the system temperature and the particle density are both lower and we donrsquot expect QGP formation in these regions However the pT distributions of the hadrons produced do not reflect the conditions from the early stages of the collision and are influenced by collective flow

32 Coulomb interaction at relativistic energies The asymmetry in the number of charged pions produced in heavy ion collisions at AGS and SPS energies was interpreted as an effect of Coulomb interaction between the pions produced and the positive charge from reaction partners At lower energies the colliding nuclei are fully stopped and expand relatively slowly in all directions therefore the total charge stays together for sufficient time to significantly accelerate or decelerated the charged pions produced The interaction between charged pions and net charge of protons changes the transverse momentum of pions with the Coulomb ldquokickrdquo pc [11]

cT

cT

pppp

Tmm

minus+

⎟⎟⎠

⎞⎜⎜⎝

⎛ minus=

+perp

minusperp

+

minus

+

minus

expππ

ππ

(2)

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

3

where ( )22cT ppmm plusmn+=plusmn

perp At higher energies the colliding nuclei are no longer stopped the system expands faster in longitudinal direction (a higher degree of transparency) resulting in a smaller Coulomb effect

The ratio of negative to positive pions produced in Au+Au collisions at two different energies

NNs = 5 GeV (at E866-AGS) and NNs =200 GeV (at BRAHMS-RHIC) as a function of transverse mass is shown in the left panel of the Fig 3 At low mT ndash mass the pion ratios behave differently while at high mT ndash mass both ratios approach one A pronounced enhancement in the +minus ππ ratio at low transverse kinetic energy is evident for the AGS data while the BRAHMS data are found to be almost flat with respect to mT -mass If stopping is significant a large amount of positive charge from the beam and target nuclei will concentrate around the mid-rapidity of the colliding system Therefore the AGS enhancement could be explained if low-pT positive pions are pushed towards higher pT by a large net positive charge at mid-rapidity conversely negative pions would be attracted At RHIC energy a low net-baryon density is observed at midrapidity [14] and consequently there are negligible Coulomb effects

Figure 3 Left AGS Au+Au at 116 AGeVc (full symbols) [12] and BRAHMS Au+Au at 200 GeV [13] (red full symbols are for 0-10 centrality blue open symbols for 40-60 centrality) Right Charged pion ratio produced in 0-10 Au+Au collisions at 200 GeV [13] as a function of transverse momentum The lines are the calculations using the relation (2) for pc=2 MeVc (blue) pc=5 MeVc (red) and pc=8 MeVc (green)

In the right panel of Fig 3 the BRAHMS negative to positive charged pions ratio for the most central 0-10 Au+Au collisions at NNs =200 GeV is plotted as a function of transverse momentum is compared to the ratio predicted by Eq (2) using three constant values of Coulomb kick 2 MeVc 5 MeVc and 8 MeVc

33 High pT suppression The nuclear modification factor RAA for unidentified charged hadrons as function of pT for the most central Au+Au collisions at pseudorapidities 0 08 26 30 and 36 is shown in Figure 4 The error bars are statistical the shaded bands are the systematic errors The shaded band around unity shows the systematic uncertainty in the number of binary collisions The RAA values decrease for pTgt2 GeVc showing the suppression of the charged hadron yields relative to the p+p reference At high pT (pT gt4 GeVc) the charged hadron yields are suppressed by a factor of ~ 3 as compared with binary scaled p+p yields For all the studied pseudorapidities the RAA distribution remains systematically lower than unity for central collisions and shows a slight decrease of the high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This suppression has been interpreted as due to

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

4

the energy loss of the energetic partons traversing the hot and dense medium produced The high pT suppression observed in the most central 200 GeV Au+Au collisions persists over a wide range in pseudorapidity and it is almost constant for all the angles studied

The nuclear modification factors for particles identified show distinct mesonbaryon dependence Nuclear modification factors for charged pions protons and antiprotons produced in Au+Au collisions at NNs =200 GeV for five rapidities y = 00 08 26 31 34 are presented in the Figure 5 Error bars represent statistical errors The shaded boxes around points show systematic errors The dotted lines indicate the expectation of binary scaling The shaded band around unity indicates the systematic error associated with the uncertainty in the number of binary collisions

Figure 4 Nuclear modification factor RAA for unidentified charged hadrons at different pseudorapidities for 200 GeV Au+Au collisions (0-10 centrality) Figure taken from ref [16]

For all the studied rapidities the pion RAA distribution remains systematically lower than unity for

most central Au+Au collisions at NNs =200 GeV At high pT the charged pion yields are suppressed by a factor of ~ 5 as compared with binary scaled p+p pion yields The suppression of pions at high pT compared with p+p collisions indicates that the partons undergo a large energy loss due to a hot dense medium created during the collisions The RAA shows constant high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This result could be an indication that other nuclear effects than parton energy loss might contribute to the rapidity constant suppression [15]

Figure 5 Nuclear modification factor RAA for pions protons and antiprotons at different rapidity for the most central collisions (0-10 centrality) Figure taken from ref [16]

In contrast the proton and antiproton yields do not show suppression with respect to binary scaling in the intermediate pT range for all the rapidities Due to the poor statistics for rapidity y=26 which corresponds to the FS positioned at 80 relative to the beam line we present only the RAA for protons

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

5

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6

where ( )22cT ppmm plusmn+=plusmn

perp At higher energies the colliding nuclei are no longer stopped the system expands faster in longitudinal direction (a higher degree of transparency) resulting in a smaller Coulomb effect

The ratio of negative to positive pions produced in Au+Au collisions at two different energies

NNs = 5 GeV (at E866-AGS) and NNs =200 GeV (at BRAHMS-RHIC) as a function of transverse mass is shown in the left panel of the Fig 3 At low mT ndash mass the pion ratios behave differently while at high mT ndash mass both ratios approach one A pronounced enhancement in the +minus ππ ratio at low transverse kinetic energy is evident for the AGS data while the BRAHMS data are found to be almost flat with respect to mT -mass If stopping is significant a large amount of positive charge from the beam and target nuclei will concentrate around the mid-rapidity of the colliding system Therefore the AGS enhancement could be explained if low-pT positive pions are pushed towards higher pT by a large net positive charge at mid-rapidity conversely negative pions would be attracted At RHIC energy a low net-baryon density is observed at midrapidity [14] and consequently there are negligible Coulomb effects

Figure 3 Left AGS Au+Au at 116 AGeVc (full symbols) [12] and BRAHMS Au+Au at 200 GeV [13] (red full symbols are for 0-10 centrality blue open symbols for 40-60 centrality) Right Charged pion ratio produced in 0-10 Au+Au collisions at 200 GeV [13] as a function of transverse momentum The lines are the calculations using the relation (2) for pc=2 MeVc (blue) pc=5 MeVc (red) and pc=8 MeVc (green)

In the right panel of Fig 3 the BRAHMS negative to positive charged pions ratio for the most central 0-10 Au+Au collisions at NNs =200 GeV is plotted as a function of transverse momentum is compared to the ratio predicted by Eq (2) using three constant values of Coulomb kick 2 MeVc 5 MeVc and 8 MeVc

33 High pT suppression The nuclear modification factor RAA for unidentified charged hadrons as function of pT for the most central Au+Au collisions at pseudorapidities 0 08 26 30 and 36 is shown in Figure 4 The error bars are statistical the shaded bands are the systematic errors The shaded band around unity shows the systematic uncertainty in the number of binary collisions The RAA values decrease for pTgt2 GeVc showing the suppression of the charged hadron yields relative to the p+p reference At high pT (pT gt4 GeVc) the charged hadron yields are suppressed by a factor of ~ 3 as compared with binary scaled p+p yields For all the studied pseudorapidities the RAA distribution remains systematically lower than unity for central collisions and shows a slight decrease of the high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This suppression has been interpreted as due to

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

4

the energy loss of the energetic partons traversing the hot and dense medium produced The high pT suppression observed in the most central 200 GeV Au+Au collisions persists over a wide range in pseudorapidity and it is almost constant for all the angles studied

The nuclear modification factors for particles identified show distinct mesonbaryon dependence Nuclear modification factors for charged pions protons and antiprotons produced in Au+Au collisions at NNs =200 GeV for five rapidities y = 00 08 26 31 34 are presented in the Figure 5 Error bars represent statistical errors The shaded boxes around points show systematic errors The dotted lines indicate the expectation of binary scaling The shaded band around unity indicates the systematic error associated with the uncertainty in the number of binary collisions

Figure 4 Nuclear modification factor RAA for unidentified charged hadrons at different pseudorapidities for 200 GeV Au+Au collisions (0-10 centrality) Figure taken from ref [16]

For all the studied rapidities the pion RAA distribution remains systematically lower than unity for

most central Au+Au collisions at NNs =200 GeV At high pT the charged pion yields are suppressed by a factor of ~ 5 as compared with binary scaled p+p pion yields The suppression of pions at high pT compared with p+p collisions indicates that the partons undergo a large energy loss due to a hot dense medium created during the collisions The RAA shows constant high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This result could be an indication that other nuclear effects than parton energy loss might contribute to the rapidity constant suppression [15]

Figure 5 Nuclear modification factor RAA for pions protons and antiprotons at different rapidity for the most central collisions (0-10 centrality) Figure taken from ref [16]

In contrast the proton and antiproton yields do not show suppression with respect to binary scaling in the intermediate pT range for all the rapidities Due to the poor statistics for rapidity y=26 which corresponds to the FS positioned at 80 relative to the beam line we present only the RAA for protons

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

5

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6

the energy loss of the energetic partons traversing the hot and dense medium produced The high pT suppression observed in the most central 200 GeV Au+Au collisions persists over a wide range in pseudorapidity and it is almost constant for all the angles studied

The nuclear modification factors for particles identified show distinct mesonbaryon dependence Nuclear modification factors for charged pions protons and antiprotons produced in Au+Au collisions at NNs =200 GeV for five rapidities y = 00 08 26 31 34 are presented in the Figure 5 Error bars represent statistical errors The shaded boxes around points show systematic errors The dotted lines indicate the expectation of binary scaling The shaded band around unity indicates the systematic error associated with the uncertainty in the number of binary collisions

Figure 4 Nuclear modification factor RAA for unidentified charged hadrons at different pseudorapidities for 200 GeV Au+Au collisions (0-10 centrality) Figure taken from ref [16]

For all the studied rapidities the pion RAA distribution remains systematically lower than unity for

most central Au+Au collisions at NNs =200 GeV At high pT the charged pion yields are suppressed by a factor of ~ 5 as compared with binary scaled p+p pion yields The suppression of pions at high pT compared with p+p collisions indicates that the partons undergo a large energy loss due to a hot dense medium created during the collisions The RAA shows constant high pT suppression with respect to p+p collisions going from midrapidity to forward rapidity This result could be an indication that other nuclear effects than parton energy loss might contribute to the rapidity constant suppression [15]

Figure 5 Nuclear modification factor RAA for pions protons and antiprotons at different rapidity for the most central collisions (0-10 centrality) Figure taken from ref [16]

In contrast the proton and antiproton yields do not show suppression with respect to binary scaling in the intermediate pT range for all the rapidities Due to the poor statistics for rapidity y=26 which corresponds to the FS positioned at 80 relative to the beam line we present only the RAA for protons

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

5

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6

The RAA distributions for protons and antiprotons are within errors approximately independent of the rapidity

Conclusions Pion and kaon production are enhanced as a function of collision centrality Different behaviour for pion and kaon enhancement factors at forward rapidity was observed At RHIC energies the Coulomb interaction is negligible The high pT suppression observed in Au+Au collisions at NNs =200 GeV is almost independent of rapidity but does depend on the particle type This might indicate an interplay between (partonic) energy loss and hydrodynamics

Acknowledgments We thank the BRAHMS Collaboration for their excellent and dedicated work to acquire and process the unique experimental data and their support to our group This work was partially supported by a grant of the Romanian National Authority for Scientific Research CNS-UEFISCDI project number 3405102011 The work of Oana Ristea and Catalin Ristea was also supported by the strategic grant POSDRU8915S58852 Project bdquoPostdoctoral programme for training scientific researchersrdquo co-financed by the European Social Found within the Sectorial Operational Program Human Resources Development 2007-2013

References [1] Jipa A and Besliu C 2002 Elemente de Fizica Nucleara Relativista Note de curs ed

Universitatii Bucuresti Beşliu C et al 2000 NuclPhysA 672 446 Jipa Al 1996 JPhysG NuclPartPhys 22 231

[2] Shuryak E 2009 NuclPhysProcSuppl195 111 [3] Blaizot J P 2001 Preprint httparxivorgabshep-ph01071311 [4] Rafelski J and Letessier J 2006 EurPhysJA29 107 [5] BRAHMS Collaboration 2003 Nucl Instr Meth A499 437 [6] wwwbnlgovRHIC [7] Rafelski J 1982 Phys Rept 88 331 Koch P Muller B and Rafelski J 1986 Phys Rept 142 167 [8] Sorge H 1995 Phys Rev C 52 3291 [9] STAR Collaboration 2011 Phys Rev C 83 34910 [10] Van Hove L 1982 Phys Lett B 118 138 [11] Barz H W Bondorf J P Gaardhoje J J and Heiselberg H 1998 PhysRevC 57 2536 1997

PhysRevC 56 1553 [12] Ahle L et al (E802 Coll) 1998 PhysRevC 57 R466 [13] Arsene I et al (BRAHMS Coll) 2005 Phys Rev C 72 014908 [14] Bearden I et al (BRAHMS Collaboration) 2004 Phys Rev Lett 93 102301 Arsene I et al

(BRAHMS Coll) 2009 Phys Lett B 677 267 [15] Arsene I et al (BRAHMS Coll) 2007 Phys Lett B 650 219 Arsene I et al (BRAHMS Coll)

2007 Phys Rev Lett 98 252001 [16] Ristea C 2007 Ph D Thesis University of Copenhagen

Rutherford Centennial Conference on Nuclear Physics IOP PublishingJournal of Physics Conference Series 381 (2012) 012042 doi1010881742-65963811012042

6